The kernel image for QEMUv8 is Image, not zImage (there is no
compressed image support in Linux for arm64). Fix the makefile.

Fixes: c54f4f51 ("qemu, qemu_v8: do not build Linux modules")
Signed-off-by: Jerome Forissier <jerome@forissier.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>